1. Field of the Invention
The present invention relates to a laminated piezoelectric element that can be applied to a piezoelectric actuator and the like.
2. Description of the Related Art
A laminated piezoelectric element of the whole-surface electrode structure in which the internal electrode layers are covering the whole surfaces of the piezoelectric layers, produces a larger displacement than a partial electrode structure in which the internal electrode layers have portions without covering the piezoelectric layers, and permits less stress to be concentrated at the ends of the internal electrode layers. Employment of the whole-surface electrode structure, therefore, makes it possible to obtain a laminated piezoelectric element having higher displacement characteristics and higher reliability than those of the partial electrode structure.
In the case of the whole-surface electrode structure, the ends of the internal electrode layers are exposed on the whole side surfaces of the laminated piezoelectric element. The ends must be electrically conductive to one of the pair of side-surface electrodes but must be electrically insulated from the other side-surface electrode. As the laminated piezoelectric element undergoes displacement, a large stress is applied to the insulating portions between the side-surface electrodes and the internal electrode layers. Therefore, a structural contrivance is necessary for enhancing the insulation between the internal electrode layers and the side-surface electrodes. An insulating structure has heretofore been proposed according to which grooves are formed in the side surfaces of the laminated piezoelectric element of the whole-surface electrode structure, and are filled with an insulating resin (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 2001-69771, 2001-102647, 2002-111088, 2001-244513, and 2001-77436).
With the above conventional insulating structure, however, there exists a large difference in the coefficient of linear expansion between the insulating resin filled in the grooves and the piezoelectric material. Therefore, peeling occurs in the interface between the insulating resin and the piezoelectric layer or cracks occur in the insulating resin at a relatively early time in the reliability testing such as cooling/heating cycle testing. The cracks trigger a short-circuit among the internal electrode layers or between the internal electrode layers and the side-surface electrodes that are to be insulated therefrom.